Isolation of novolak resin without high temperature distillation and photoresist composition therefrom

ABSTRACT

The present invention provides a method for producing a water insoluble, aqueous alkali soluble novolak resins having consistent molecular weight and superior performance in photoresist composition, by isolating such novolak resin without high temperature distillation. A method is also provided for producing photoresist composition from such a novolak resin and for producing semiconductor devices using such a photoresist composition.

This application is a continuation of application Ser. No. 08/568,916,filed Dec. 7, 1995, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a process for producing a novolak resinhaving consistent molecular weight and high performance, and for usingsuch a novolak resin in light-sensitive compositions. The presentinvention also relates to a process for making a superior qualitylight-sensitive composition useful as a positive-working photoresist.Further, the present invention relates to a process for coatingsubstrates with these light-sensitive compositions, as well as theprocess of coating, imaging and developing these light-sensitivemixtures on such substrates.

Photoresist compositions are used in microlithography processes formaking miniaturized electronic components, such as in the fabrication ofcomputer chips and integrated circuits. Generally, in these processes, athin coating of a film of a photoresist composition is first applied toa substrate material, such as silicon wafers used for making integratedcircuits. The coated substrate is then baked to evaporate any solvent inthe photoresist composition and to fix the coating onto the substrate.The baked coated surface of the substrate is next subjected to animage-wise exposure to radiation.

This radiation exposure causes a chemical transformation in the exposedareas of the coated surface. Visible light, ultraviolet (UV) light,electron beam and X-ray radiant energy are radiation types commonly usedtoday in microlithographic processes. After this image-wise exposure,the coated substrate is treated with a developer solution to dissolveand remove either the radiation-exposed or the unexposed areas of thecoated surface of the substrate.

Novolak resins are frequently used a polymeric binder in liquidphotoresist formulations. These resins are typically produced byconducting a condensation reaction between formaldehyde and one or moremulti-substituted phenols, in the presence of an acid catalyst, such asoxalic acid, maleic acid, or maleic anhydride. In producingsophisticated semiconductor devices, it has become increasinglyimportant to provide novolak resin of superior quality in terms ofdissolution rate, better binding properties with diazonaphthoquinone,and heat resistance.

There are two types of photoresist compositions, negative-working andpositive-working. When negative-working photoresist compositions areexposed image-wise to radiation, the areas of the resist compositionexposed to the radiation become less soluble to a developer solution(e.g. a cross-linking reaction occurs) while the unexposed areas of thephotoresist coating remain relatively soluble to such a solution. Thus,treatment of an exposed negative-working resist with a developer causesremoval of the non-exposed areas of the photoresist coating and thecreation of a negative image in the coating. Thereby uncovering adesired portion of the underlying substrate surface on which thephotoresist composition was deposited.

On the other hand, when positive-working photoresist compositions areexposed image-wise to radiation, those areas of the photoresistcomposition exposed to the radiation become more soluble to thedeveloper solution (e.g. a rearrangement reaction occurs) while thoseareas not exposed remain relatively insoluble to the developer solution.Thus, treatment of an exposed positive-working photoresist with thedeveloper causes removal of the exposed areas of the coating and thecreation of a positive image in the photoresist coating. Again, adesired portion of the underlying substrate surface is uncovered.

After this development operation, the now partially unprotectedsubstrate may be treated with a substrate-etchant solution or plasmagases and the like. The etchant solution or plasma gases etch thatportion of the substrate where the photoresist coating was removedduring development. The areas of the substrate where the photoresistcoating still remains are protected and, thus, an etched pattern iscreated in the substrate material which corresponds to the photomaskused for the image-wise exposure of the radiation. Later, the remainingareas of the photoresist coating may be removed during a strippingoperation, leaving a clean etched substrate surface. In some instances,it is desirable to heat treat the remaining photoresist layer, after thedevelopment step and before the etching step, to increase its adhesionto the underlying substrate and its resistance to etching solutions.

Positive working photoresist compositions are currently favored overnegative working resists because the former generally have betterresolution capabilities and pattern transfer characteristics.Photoresist resolution is defined as the smallest feature which theresist composition can transfer from the photomask to the substrate witha high degree of image edge acuity after exposure and development. Inmany manufacturing applications today, resist resolution on the order ofless than one micron are necessary. In addition, it is almost alwaysdesirable that the developed photoresist wall profiles be near verticalrelative to the substrate. Such demarcations between developed andundeveloped areas of the resist coating translate into accurate patterntransfer of the mask image onto the substrate.

DESCRIPTION OF THE PRIOR ART

In the recent years there has been significant progress in novolak resinsynthesis. It has been reported that under vigorous synthetic conditionsthe structure of novolak resin changes, especially when highconcentration of acid catalyst and high temperature is used, Rahman etal, "Rearrangement of Novolak Resin", presented at SPIE conference,1994; Khadim et al "The nature and Degree of Substitution Patterns inNovolaks by Carbon-13 NMR Spectroscopy", presented at SPIE conference,1993. In a typical novolak reaction, a reactor is charged with phenoliccompounds, an acid catalyst such as oxalic acid, maleic acid, p-toluenesulphonic acid or any mineral acid, and heated to about 95° to 100° C.Formaldehyde is slowly added and the mixture is heated at reflux forabout 6 hours. At the end of the condensation period, the reactor isconverted to distillation, and the temperature is raised to about 200°C. At this point vacuum is slowly drawn, the temperature is raised toabout 220° C., and the pressure is reduced to below about 20 mm Hg.After the volatiles have been distilled off, the vacuum is released andthe molten resin collected and allowed to cool. During the course of theabove resin synthesis sequence, samples were taken at varioustemperatures and inspected by GPC (Gas Phase Chromotography). It wasfound that there was a decrease of the weight average molecular weight(all molecular weights are weight average unless otherwise specified) ofthe polymer, especially in the temperature range between about 160°-190°C. (Rahman et al, "The Effect of Lewis Bases on the Molecular Weight ofNovolak Resins", presented at Ellenville Conference, 1994). Themolecular weight decrease was not observed unless the phenolic compoundsare extremely pure. If the phenolic compounds contains a trace amount ofnitrogen base, the molecular weight decrease during the distillationprocess was not observed. In copending U.S. patent applications Ser. No.997,942, filed Dec. 29, 1992 (WO 94/14862) and Ser. No. 999,500, filedDec. 29, 1992 (WO 94/14863), assigned to the same assignee as thesubject application and incorporated herein by reference, an improvedprocess is disclosed to control molecular weight by adjusting the amountof Lewis base in the phenolic compounds before or after thecondensation. It was disclosed that during the purification process ofsuch phenolic compounds using an ion exchange resin, distillation,and/or a solvent extraction process, to remove metal ions, a minoramount of base present was also removed. Due to the absence of thisbase, the novolak resin was partially depolymerized during themanufacturing process. The physical properties of the depolymerizedresin changed due to degradation and it was not useful for photoresistcompositions. This problem can be substantially avoided by adjusting thelevel of Lewis base before or after the condensation step of the novolakmanufacturing process.

In copending U.S. patent application Ser. No. 366,634, filed on Dec. 30,1994, assigned to the same assignee as the subject application andincorporated herein by reference, an improved process is disclosed forisolating a novolak resin at a temperature less than about 140° C. byusing subsurface forced steam distillation to avoid high temperaturemolecular weight breakdown of the resin. It is known that an alkalisoluble novolak resin can be made by the condensation reaction of amixture of phenolic monomers with an aldehyde source. Such novolak resinsynthesis processes are disclosed in U.S. Pat. No. 5,346,799,incorporated herein by reference.

SUMMARY OF THE INVENTION

It has now been found that the novolak resin synthesis process can befurther improved by isolating the phenol formaldehyde condensationproduct at room temperature without any high temperature distillation.This can be accomplished by adding a water soluble organic solvent, suchas acetone or a C₁ -C₃ allyl alcohol, such as methanol, to thephenol-formaldehyde condensation product, followed by addition of water,preferably DI (deionized) water, to thereby precipitate the condensationproduct, leaving behind the unreacted phenols, and useless oligomers inthe water soluble solvent/water solution. This precipitate can then bedissolved in a suitable novolak resin solvent such as PGMEA (propyleneglycol methyl ether acetate), 2-heptanone or ethyl lactate, or it can bedried, such as in a vacuum drier.

The present invention relates to a process for producing novolak resinof consistent molecular weight having high performance and for usingsuch a novolak resin in light-sensitive compositions. The invention alsorelates to a photoresist composition containing such a novolak resin andto a process for producing such photoresist compositions. The inventionfurther relates to semiconductor devices using such photoresistscontaining these novolak resin and a photosensitizer, and to a processfor using such photoresists in producing semiconductor devices.

Water insoluble, aqueous alkali soluble, film forming novolak resinshaving a consistent molecular weight and high performance may beobtained by condensing formaldehyde having a very low level of metalions with one or more phenolic compounds, such as m-cresol, p-cresol,2,4 and 2,5-dimethylphenol, 3,5-dimethylphenol, and2,3,5-trimethylphenol, having a very low level of metal ions. Thecondensation reaction is preferably carried out in the presence of anacid catalyst, such as oxalic acid, maleic acid, maleic anhydride,p-toluene sulphonic acid or sufuric acid.

In the process of the present invention, the precipitate is eitherdissolved in a novolak resin solvent and the residual water is removed,such as by low temperature vacuum distillation, to obtain a novolakresin having a very consistent molecular weight with superiorperformance in photoresist compositions. The precipitate may also bedried, such as in a vacuum oven, to remove residual water and obtainsuch a novolak resin.

The present invention provides a process which comprises:

a) condensing formaldehyde with one or more phenolic compounds, in thepresence of an acid catalyst, and after the condensation reaction iscomplete, adding a water soluble solvent, such as acetone or a C₁ -C₃alkyl alcohol, such as methanol;

b) precipitating the condensation product by adding water, preferably DIwater, and removing the liquid, such as by decanting, and thereby alsoremoving the unreacted phenolic compounds;

c) dissolving the precipitate in a suitable novolak resin solvent ,andremoving the residual water, such as by low temperature vacuumdistillation, thereby producing a water insoluble, aqueous alkalisoluble, film forming novolak resin.

The present invention further provides a process for producing apositive photoresist composition having a superior performance. Thesubject process comprises:

a) condensing formaldehyde with one or more phenolic compounds, in thepresence of an acid catalyst, and after the condensation reaction iscomplete, adding a water soluble solvent, such as acetone or, a C₁ -C₃alkyl alcohol, such as methanol;

b) precipitating the condensation product by adding water, preferably DIwater, and removing the liquid, such as by decanting, and thereby alsoremoving the unreacted phenolic compounds;

c) dissolving the precipitate in a suitable novolak resin solvent, andremoving the residual water, such as by low temperature vacuumdistillation, thereby producing a water insoluble, aqueous alkalisoluble, film forming novolak resin.

d) providing an admixture of: 1) a photosensitive component in an amountsufficient to photosensitize the photoresist composition; 2) the waterinsoluble, aqueous alkali soluble, film forming novolak resin from stepc); and 3) a suitable photoresist solvent, and thereby forming aphotoresist composition.

The invention further provides a method for producing a semiconductordevice by producing a photo-image on a substrate by coating a suitablesubstrate with a positive working photoresist. The subject processcomprises:

a) condensing formaldehyde with one or more phenolic compounds, in thepresence of an acid catalyst, and after the condensation reaction iscomplete, adding a water soluble solvent, such as acetone or a C₁ -C₃alkyl alcohol, such as methanol;

b) precipitating the condensation product by adding DI water, andremoving the liquid, such as by decanting, and thereby also removing theunreacted phenolic compounds;

c) dissolving the precipitate in a suitable novolak resin solvent andremoving the residual water, such as by low temperature vacuumdistillation, thereby producing a water insoluble, aqueous alkalisoluble, film forming novolak resin having a consistent molecular weightand superior quality;

d) providing an admixture of: 1) a photosensitive component in an amountsufficient to photosensitive the photoresist composition; 2) the waterinsoluble, aqueous alkali soluble, film forming novolak resin and 3) asuitable photoresist solvent and thereby forming a photoresistcomposition;

e) coating a suitable substrate with the photoresist composition;

f) heat treating the coated substrate until substantially all of thephotoresist solvent is removed; image-wise exposing the photosensitivecomposition and removing the image-wise exposed areas of suchcomposition with a suitable developer, such as an aqueous alkalinedeveloper. Optionally one may also perform a baking of the substrateeither immediately before or after the removing step.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Novolak resins have been commonly used in the art of photoresistmanufacture as exemplified by "Chemistry and Application of PhenolicResins", Knop A. And Scheib, W.; Springer Verlag, New York, 1979 inChapter 4. Similarly, o-quinone diazides are well known to the skilledartisan as demonstrated by "Light Sensitive Systems", Kosar, J.; JohnWiley & Sons, New York, 1965 Chapter 7.4. However, the instant inventionhas found that the use of particular resins isolated without hightemperature distillation, as opposed to those taught in the prior art,produces a photoresist having a superior resolution and depth of focus.

The present invention provides a novolak resin, a photoresistcomposition containing such a novolak resin and a process for producingsemiconductor devices using such a photoresist composition. Thephotoresist composition is formed by providing an admixture of aphotosensitizer, the subject water insoluble, aqueous alkali solublenovolak resin and a suitable photoresist solvent. Suitable solvents forsuch photoresists and for novolak resins may include propylene glycolmono-alkyl ether, propylene glycol alkyl (e.g. methyl) ether acetate,ethyl-3-ethoxypropionate, ethyl lactate, mixtures ofethyl-3-ethoxypropionate and ethyl lactate, butyl acetate, xylene,diglyme, ethylene glycol monoethyl ether acetate. The preferred solventsare propylene glycol methyl ether acetate (PGMEA), ethyl lactate,2-heptanone, and ethyl-3-ethoxypropionate (EEP).

Other optional ingredients such as colorants, dyes, anti-striationagents, leveling agents, plasticizers, adhesion promoters, speedenhancers, solvents and such surfactants as non-ionic surfactants may beadded to the solution of novolak resin, sensitizer and solvent beforethe photoresist composition is coated onto a substrate. Examples of dyeadditives that may be used together with the photoresist compositions ofthe present invention include Methyl Violet 2B (C.I. No. 42535), CrystalViolet (C.I. 42555). Malachite Green (C.I. No. 42000), Victoria Blue B(C.I. No. 44045) and Neutral Red (C.I. No. 50040) at one to ten percentweight levels, based on the combined weight of novolak and sensitizer.The dye additives help provide increased resolution by inhibiting backscattering of light off the substrate.

Anti-striation agents may be used at up to about a five percent weightlevel, based on the combined weight of novolak and sensitizer.Plasticizers which may be used include, for example, phosphoric acidtri-(beta-chloroethyl)-ester; stearic acid; dicamphor; polypropylene;acetal resins; phenoxy resins; and alkyl resins, at about one to tenpercent weight levels, based on the combined weight of novolak andsensitizer. The plasticizer additives improve the coating properties ofthe material and enable the application of a film that is smooth and ofuniform thickness to the substrate.

Adhesion promoters which may be used include, for example,beta-(3,4-epoxycyclohexyl)-ethyltrimethoxysilane;p-methyl-disilane-methyl methacrylate; vinyl trichlorosilane; andgamma-amino-propyl triethoxysilane, up to about a 4 percent weightlevel, based on the combined weight of novolak and sensitizer.Development speed enhancers that may be used include, for example,picric acid, nicotinic acid or nitrocinnamic acid up to about a 20percent weight level, based on the combined weight of novolak andsensitizer. These enhancers tend to increase the solubility of thephotoresist coating in both the exposed and unexposed areas, and thusthey are used in applications when speed of development is theoverriding consideration even though some degree of contrast may besacrificed; i.e., while the exposed areas of the photoresist coatingwill be dissolved more quickly by the developer, the speed enhances willalso cause a larger loss of photoresist coating from the unexposedareas.

The solvents may be present in the overall composition in an amount ofup to 95% by weight of the solids in the composition. Solvents, ofcourse are substantially removed after coating of the photoresistsolution on a substrate and subsequent drying. Non-ionic surfactantsthat may be used include, for example, nonylphenoxy poly(ethyleneoxy)ethanol; octylphenoxy ethanol at up to about 10% weight levels, based onthe combined weight of novolak and sensitizer.

The prepared photoresist solution, can be applied to a substrate by anyconventional method used in the photoresist art, including dipping,spraying, whirling and spin coating. When spin coating, for example, theresist solution can be adjusted with respect to the percentage of solidscontent, in order to provide coating of the desired thickness, given thetype of spinning equipment utilized and the amount of time allowed forthe spinning process. Suitable substrates include silicon, aluminum,polymeric resins, silicon dioxide, doped silicon dioxide, siliconnitride, tantalum, copper, polysilicon, ceramics, aluminum/coppermixtures; gallium arsenide and other such Group III/V compounds.

The photoresist coatings produced by the described procedure areparticularly suitable for application to thermally grown silicon/silicondioxide-coated wafers, such as are utilized in the production ofmicroprocessors and other miniaturized integrated circuit components. Analuminum/aluminum oxide wafer can also be used. The substrate may alsocomprise various polymeric resins, especially transparent polymers suchas polyesters. The substrate may have an adhesion promoted layer of asuitable composition, such as one containing hexa-alkyl disilazane.

The photoresist composition solution is then coated onto the substrate,and the substrate is treated at a temperature from about 70° C. to about110° C. for from about 30 seconds to about 180 seconds on a hot plate orfor from about 15 to about 90 minutes in a convection oven. Thistemperature treatment is selected in order to reduce the concentrationof residual solvents in the photoresist, while not causing substantialthermal degradation of the photosensitizer. In general, one desires tominimize the concentration of solvents and this first temperaturetreatment is conducted until substantially all of the solvents haveevaporated and a thin coating of photoresist composition, on the orderof one micron in thickness, remains on the substrate. In a preferredembodiment the temperature is from about 85° C. to about 95° C. Thetreatment is conducted until the rate of change of solvent removalbecomes relatively insignificant. The temperature and time selectiondepends on the photoresist properties desired by the user, as well asthe equipment used and commercially desired coating times. The coatedsubstrate can then be exposed to actinic radiation, e.g., ultravioletradiation, at a wavelength of from about 300 nm to about 450 nm, x-ray,electron beam, ion beam or laser radiation, in any desired pattern,produced by use of suitable masks, negatives, stencils, templates, etc.

The photoresist is then optionally subjected to a post exposure secondbaking or heat treatment either before or after development. The heatingtemperatures may range from about 90° C. to about 120° C., morepreferably from about 100° C. to about 110° C. The heating may beconducted for from about 30 seconds to about 2 minutes, more preferablyfrom about 60 seconds to about 90 seconds on a hot plate or about 30 toabout 45 minutes by convection oven.

The exposed photoresist-coated substrates are developed to remove theimage-wise exposed areas by immersion in an alkaline developing solutionor developed by spray development process. The solution is preferablyagitated, for example, by nitrogen burst agitation. The substrates areallowed to remain in the developer until all, or substantially all, ofthe photoresist coating has dissolved from the exposed areas. Developersmay include aqueous solutions of ammonium or alkali metal hydroxides.One preferred hydroxide is tetramethyl ammonium hydroxide. After removalof the coated wafers from the developing solution, one may conduct anoptional post-development heat treatment or bake to increase thecoating's adhesion and chemical resistance to etching solutions andother substances. The post-development heat treatment can comprise theoven baking of the coating and substrate below the coating's softeningpoint. In industrial applications, particularly in the manufacture ofmicrocircuitry units on silicon/silicon dioxide-type substrates, thedeveloped substrates may be treated with a buffered, hydrofluoric acidbase etching solution. The photoresist compositions of the presentinvention are resistant to acid-base etching solutions and provideeffective protection for the unexposed photoresist-coating areas of thesubstrate.

The following specific examples will provide detailed illustrations ofthe methods of producing and utilizing compositions of the presentinvention. These examples are not intended, however, to limit orrestrict the scope of the invention in any way and should not beconstrued as providing conditions, parameters or values which must beutilized exclusively in order to practice the present invention.

EXAMPLE 1

250 grams of phenolic compounds consisting of 5.0 moles of m-cresol and3.0 moles of 3,5-xylenol were transferred to a four necked flaskequipped with a condenser, a thermometer, and a dropping funnel. 2.5grams of oxalic acid (1% by weight of the phenols) was added and theflask was heated to 95° C. 123.0 g of formaldehyde (molar ratio ofphenols/formaldehyde 1/0.685) was added dropwise over one and a halfhours. The reaction was allowed to continue for 6 hours at 95° C. 600grams of methanol was added and stirred for 10 minutes. 92.7 grams of DIwater (11.1%, by weight, of the batch) was added with stirring over aperiod often minutes. Stirring was stopped and a white precipitatesettled to the bottom of the flask. The liquid was sucked out from thetop of a second flask into which the precipitate was placed. Theprecipitate was collected as fraction A in a beaker and dried in avacuum oven. 80 grams of water was added to the liquid in the secondflask and fraction B was separated in the same manner as fraction A. Theprocess was continued and the fractions C and D were separated in thesame manner. All the fractions were then dried in a vacuum drier. Theunreacted phenols and very low molecular weight oligomers remained inthe methanol water solution. The analytical results are shown in Table 1below.

                  TABLE 1    ______________________________________    Fractions            Weight (g)                     GPC MW    PD  % m-Cresol                                           % 3,5-Xyl    ______________________________________    B       24.87    8733      5.5 2.3     <0.1    C       72.43    7158      3.8 2.1     <0.1    D       23.49    3162      1.9 1.7     <0.1    E       23.37    1701      1.6 2.3     <0.1    ______________________________________

EXAMPLE 2

150 grams of phenolic compounds consisting of 6.0 moles of m-cresol, 2.5moles of 3,5-xylenol and 0.5 moles of 2,3,5-xylenol were transferred toa four necked flask equipped with a condenser, a thermometer, and adropping funnel. 1.125 grams of oxalic acid (0.75% by weight of thephenols) was added and the flask was heated to 95° C. 70.7 g offormaldehyde (molar ratio of phenols/formaldehyde 1/0.66) was addeddropwise over one and a half hours. The reaction was allowed to continuefor 6 hours at 95° C. 329 grams of methanol was added and stirred for 10minutes. 175 grams of DI water (43.7% of the batch) was added and duringten minutes and stirred for 30 minutes. Stirring was stopped and a whiteprecipitate was settled at the bottom of the flask. The liquid wassucked out from the top and discarded. The precipitate was collected ina beaker and dried in a vacuum oven.

EXAMPLE 4

A 50 gram photoresist test sample was prepared according to thefollowing formulation:

    ______________________________________    NK-280 (a proprietary 2,1,5-diazonaphthoquinone                               2.0218  gm    sulfonyl chloride based sensitizer from Nippon Zeon Co)    NK-240 (a proprietary 2,1,4-diazonaphthoquinone                               0.8382  gm    sulphonyl chloride based sensitizer from Nippon Zeon Co.)    Novolak Resin fraction C from example 1                               1.3545  gm    Novolak Resin fraction D from example 1                               1.35345 gm    Novolak Resin fraction E from example 1                               4.0634  gm    Pyrogallol from Aldrich Co.                               0.1630  gm    BI26X-SA (a proprietary speed enhancer                               1.203   gm    from Nippon Zeon Co.)    KP-341, a striantion free surfactant                               0.004   gm    from Shinetsue Chem. Co. (2% in Ethyl Lactate)    Ethyl Lactate              33.147  gm    n-Butyl Acetate            5.849   gm    ______________________________________

The resist sample was coated on a hexamethyldisilazane (HMDS) primedsilicon wafer to 1.083 μm thickness using a soft bake at 90° C. for 60seconds on an I-line hot plate (SVG® 8100). The exposure matrix wasprinted on the coated wafers using a 0.54 NA NIKON® i-line stepper and aNIKON® resolution reticle. The exposed wafers were PEB (post exposurebaked) at 110° C. for 70 seconds on a in line hot plate. The wafers werethen developed using AZ® 300 MIF TMAH (tetramethyl ammoniumhydroxide--2.38%) developer. The developed wafers were examined using aHITACHI® S400 SEM (scanning electron microscope). A nominal dose (Doseto Print, DTP) is measured at the best focus, the dose required toprecisely replicate a given feature. Resolution and depth of focus (DOF)were measured and are shown in Table 2 below.

EXAMPLES 5

A 50 gram photoresist test sample was prepared according to thefollowing formulation:

    ______________________________________    NK-280 (a proprietary 2,1,5-diazonaphthoquinone                               2.0218  gm    sulfonyl chloride based sensitizer from Nippon Zeon Co)    NK-240 (a proprietary 2,1,4-diazonaphthoquinone                               0.8382  gm    sulphonyl chloride based sensitizer from Nippon Zeon Co.)    Novolak Resin fraction D from example 1                               1.437   gm    Novolak Resin fraction E from example 1                               5.336   gm    Pyrogallol from Aldrich Co.                               0.1630  gm    BI26X-SA (a proprietary speed enhancer                               1.203   gm    from Nippon Zeon Co.)    KP-341, a striantion free surfactant                               0.004   gm    from Shinetsue Chem. Co. (2% in Ethyl Lactate)    Ethyl Lactate              33.147  gm    n-Butyl Acetate            5.849   gm    ______________________________________

The photoresist resist sample was coated on a hexamethyldisilazane(HMDS) primed silicon wafer to 1.083 μm thickness using a soft bake at90° C. for 60 seconds on an I-line hot plate (SVG® 8100). The exposurematrix was printed on the coated wafers using a 0.54 NA NIKON® i-linestepper and a NIKON® resolution reticle. The exposed wafers were PEB(post exposure baked) at 110° C. for 70 seconds on a in line hot plate.The wafers were then developed using AZ® 300 MIF TMAH (tetramethylammonium hydroxide--2.38%) developer. The developed wafers were examinedusing a HITACHI® S-400 SEM (scanning electron microscope). A nominaldose (Dose to Print, DTP) is measured at the best focus, the doserequired to precisely replicate a given feature. Resolution and depth offocus (DOF) were measured and are shown in Table 2 below.

                  TABLE 2    ______________________________________    Example #           Resin From       DTP    Resolution                                           DOF    ______________________________________    4      Example 1; fractions C, D,                            105    0.36    (-.6/.4)           & E    5      Example 1; fractions D & E                            80     0.36    (-.6/.4)    ______________________________________

COMPARATIVE EXAMPLE 1

250 grams of phenolic compounds consisting of 5.0 moles of m-cresol and3.0 moles of 3,5-xylenol were transferred to a four necked flaskequipped with a condenser, a thermometer, and a dropping funnel. 2.5grams of oxalic acid (1% by weight of the phenols) was added and theflask was heated to 95° C. 123.0 g of formaldehyde (molar ratio ofphenols/formaldehyde 1/0.685) was added dropwise over one and a halfhours. The reaction was allowed to continue for 6 hours at 95° C. Thereaction mixture was distilled up to 190° C. to remove water andsolvent, and then distilled under vacuum to 210° C. to remove unreactedphenols. The molted resin was collected in an aluminum tray. Aphotoresist formulation was made exactly as in example #4, exceptnovolak resin is used from comparative example #1 and studied as example#4. It was found that the performance of the resist(Table 3) is not goodas compare to example #4 and #5.

                  TABLE 3    ______________________________________    Example #            DTP       Resolution      DOF    ______________________________________    Comparative            160       Pattern falling over at 0.38                                      (-.6/-0.8)    example #1        micron    ______________________________________

We claim:
 1. A method for producing a positive photoresist compositionconsisting essentially of:a) condensing formaldehyde with one or morephenolic compounds, in the presence of an acid catalyst and after thecondensation reaction is complete adding a water soluble solvent; b)precipitating the condensation product by adding water, and removing thephenolic compounds, and oligomers in a water soluble solvent/watersolution, by decanting and thereby removing the unreacted phenoliccompounds and oligomers with said liquid; c) dissolving the precipitateof step b) in a novolak resin solvent selected from the group consistingof propylene glycol methyl ether acetate, 2-heptanone, ethyl lactate andethyl-3-ethoxypropionate, and removing the residual water, therebyproducing a water insoluble, aqueous alkali soluble, film formingnovolak resin; d) providing an admixture of: 1) a photosensitivecomponent in an amount sufficient to photosensitive the photoresistcomposition; 2) the water insoluble, aqueous alkali soluble, filmforming novolak resin and 3) a suitable photoresist solvent, and therebyforming a photoresist composition.
 2. The method of claim 1 where insaid acid catalyst is oxalic acid, maleic acid, maleic anhydride,sulfuric acid, or p-toluene sulphonic acid.
 3. The method of claim 1wherein the water soluble solvent is acetone or a C₁ -C₃ alkyl alcohol.4. The method of claim 1 wherein the water soluble solvent is methanol.